1. Field of the Invention
This invention concerns a thin film electroluminescent display device having two layers of electroluminescent films, and capable of multicolor display by causing these electroluminescent films to emit light respectively.
2. Description of the Prior Art
Thin film electroluminescent display devices (hereinafter simply referred to as EL display device) have been applied to the display of various types of devices in recent years. A conventional thin film EL display device in the prior art usually comprises a 6-layered structure, in which a transparent conductive film, an insulation film, an EL emission film, an insulation film and an opposing electrode film are successively laminated on a transparent substrate. The thin film EL display device is adapted such that when an alternating electric field from several tens Hz to several KHz is applied between the transparent conductive film and the opposing electrode film, ions of activated species in the EL emission film are excited to emit light. However, in the above-mentioned thin film EL display device, it has only been possible to obtain a single display color determined by the material or the like for the EL emission film.
In view of the above, a thin film EL display device for enabling multicolor display by disposing two layers of the EL emission films has been considered. The conventional EL display device having two layers of EL emission films comprises, as shown in FIG. 3, a structure in which a transparent conductive film 2, an insulation film 3, a first EL emission film 4, an insulation film 5, an intermediate electrode film 6, an insulation film 7, a second EL emission film 8, an insulation film 9 and an opposing electrode film 10 are successively laminated on a transparent glass substrate 1. When an alternating electric field is applied between the transparent conductive film 2 and the intermediate electrode film 6, the first EL emission film emits light and, when an alternating electric field is applied between the intermediate electrode film 6 and the opposing electrode film 10, the second EL emission film emits light. The multicolor display can be attained by varying the material between the first EL emission film 4 and the second EL emission film 8.
However, in the above-mentioned conventional thin film EL display device having two layers of EL emission films, it is necessary in view of the structure thereof to lead out the electrodes from three electrode films 2, 6 and 10 respectively to render the electrode leading out work complicated.
Further, an improvement has been demanded for the resolution power upon display accompanying with the increasing use as the display for various kinds of devices. However, since it is necessary that the electrode film on the side for taking out EL emission (usually the electrode film on the side of the transparent glass substrate) is a transparent conductive film and since the transparent conductive film available at the present technical level has a specific resistivity of about 2.times.20.sup.-4 .OMEGA.cm, it is considered to narrow the pattern width for improving the resolving performance upon display. However, narrowing for the pattern width leads to the increase in the conductive resistance upon leading out the electrodes. If the conduction resistance of the pattern at the transparent conductive film is increased, uneven brightness is resulted in the EL emission to degrade the display quality. Accordingly, there has been a limit for the pattern width of the transparent conductive film.